The present invention relates to a circuit for sensing the waveform of the collector current in a transistor, in particular the switching transistor of a regulated power supply operating in current mode.
The use of power switching transistors in circuits of the so-called current-mode type, in which the waveform of the collector current of the switching transistor is controlled by resistor means, is well known in the art.
A power supply of this type is shown in FIG. 1, corresponding to the device UC 1842 or UC 1846 manufactured by SGS Microelectronics. As visible, the power supply comprises a dual loop including an error amplifier EA which compares the regulated voltage V.sub.O with an externally fed reference voltage V.sub.REF and generates an error signal ES. A comparator CP is provided downstream the error amplifier EA in order to compare the error signal ES, which is fed to the non-inverting input of the comparator CP, to a saw-tooth signal RA which is proportional to the saw-tooth current which flows through the collector of the switching transistor T.sub.S. Said saw-tooth signal RA, generate by a sensing resistor R.sub.S connected between the emitter of the switching transistor T.sub.S and the ground, is fed to the inverting input of the comparator thus forming a feedback loop. The comparator CP generates a switching signal indicated by the letter C.
Block 2, connected to the collector of transistor T.sub.S, is a part of the regulated power supply and generates the regulated output voltage V.sub.O, but is not shown in detail since it is well known in the art and not necessary for the comprehension of the present invention.
In this known device, the dimensioning of the sensing resistor R.sub.S is rather difficult, since two opposite conditions must be satisfied, namely:
in order to keep the dissipated power as low as possible, the sensing resistor R.sub.S must have a low resistive value; PA1 and on the other hand the resistance of said resistor R.sub.S must be kept relatively high in order to maximize the dynamic amplitude of the saw-tooth signal.
This in turn facilitates the comparison of the saw-tooth signal RA with the error signal ES by the comparator CP.
A solution to the above mentioned problem is disclosed in the article "Current Mirror FET's cut cost and sensing losses" (Garry Fay, in EDN, Sept. 4, 1986, pages 193-200).
Said article describes a FET sensor in which a portion of the cells of the source is used for current sensing purposes, monitoring the absorbed current without intersecting in series to the current load a high-power dissipation resistor.
However, though it solves the conflicting problems of the circuit illustrated in FIG. 1, the previously described solution can be implemented exclusively in MOS technology and cannot be made in bipolar technology.